The invention relates to a hydrostatic transmission according to the kind described in more detail in the preamble of claim 1.
Hydrostatic transmissions of this type are used, for example, in construction machinery, such as excavators, for driving a mobile vehicle. A hydraulic pump is operated with a hydraulic motor in an open loop, where the hydraulic pump removes hydraulic fluid from a hydraulic fluid medium and supplies it to the hydraulic motor via a working line. Frequently a shiftable transmission is arranged downstream from the hydraulic motor. The hydraulic fluid flow leaving the hydraulic motor can be limited, using pressure control valves, so as to prevent impermissible acceleration of the vehicle when traveling downhill. The brake valve limits the hydraulic fluid flow of the hydraulic motor such that hydraulic pressure builds up on the returning working line, which together with the set intake volume of the hydraulic motor creates a brake torque. This brake torque changes with the change in intake volume of the hydraulic motor and acts upon the drive wheels as a function of the reduction of the gear arranged downstream and hence the engaged gear. Dangerous driving situations may arise if the intake volume of the hydraulic motor has been set to the minimum level and the slope descending force, which acts upon the vehicle due to grade of the hill, exceeds the possible brake torque generated by the hydraulic motor with the aid of the brake valves.
EP 0 530 842 B1 discloses a reversible hydrostatic transmission with a brake valve in which the displacement volume of the hydraulic motor is adjusted as a function of the hydraulic pressure generated by the brake valve. This way, the hydraulic motor is switched to its maximum displacement volume, generating a maximum brake torque of the hydraulic motor during travel on a steep downward slope. Since the adjustment of the hydraulic motor occurs exclusively as a function of the back pressure created by the brake valve, it is not possible to influence the deceleration created by this brake torque. Depending on the set reduction of the reducing gear arranged downstream, this deceleration also creates varying effects on the vehicle.
It is the object of the present invention to create a hydrostatic transmission, which will not lead to impermissible driving situations even in a trailing throttle mode.
The object is achieved with a generic hydrostatic transmission.